I’ve never placed a Dexcom order online. I’ve always called their toll-free number and talked to a live representative. Each time they verified my insurance coverage with a separate call to my carrier. Then they call me back and ask for a credit card number to cover any difference. I don’t think that an online transaction site could do this unless you authorize an open-ended amount.
I prefer online transactions but with my Dexcom sensors I will continue with the toll-free call. Good luck.
I always ask that they email me with a Fedex tracking number so I can follow the physical progress of the order.